Dialectric grease would not hurt anything on the back of an ignition switch, but since it isn't really exposed to the "elements" I doubt it would help except perhaps possible corrosion. Given the voltages involved, the only wire where this would be a major problem is the main power wire on the battery side. Antiseize on the spark plug threads also won't hurt anything. Do your plugs have the extra copper washer on them? If so, this is a compression ring and proper torque would be best without antiseize and even if you don't have the copper washer, the biggest problem is with "like" materials (e.g. aluminum heads, and aluminum-based spark plugs). If you have that combination I would use antiseize.
I have stock 69 351w cast iron heads so I think they have a tapered seat to seal the spark plugs instead of a crush washer? I just got the heads back from the machine shop and I have not yet purchased spark plugs. I am going to pick up some plugs on the way home from work and I am trying to decide if I should also get some anti-seize. Thanks for the dielectric info.